Estrutura, dinâmica e transições de fase do arseniato de betaína

Dissertação de Doutoramento em Física apresentada à Faculdade de Ciências da Universidade do PortoNeste trabalho apresentamos um estudo experimental relativo à sequência de transições de fase no arseniato de betaína (BA) e nos compostos parcial e fortemente deuterados (BA1-xDBAx), focando principalmente a estrutura das fases e a dinâmica dos processos envolvidos. Neste estudo foram utilizadas técnicas de difracção de raios X, de espectroscopia vibracional e técnicas macroscópicas de caracterização das propriedades dieléctricas. Os resultados apresentados nesta tese permitem clarificar alguns dos aspectos controversos do comportamento crítico do sistema BA1-xDBAx, em particular conhecer a contribuição das pontes de hidrogénio, das moléculas de betaína e dos iões arseniato nos mecanismos responsáveis pela sequência de transições de fase a baixas temperaturas. O estudo estrutural realizado a baixas temperaturas mostra claramente a existência de duas transições de fase no arseniato de betaína, que ocorrem a 119 K e a 107 K. O comportamento da intensidade dos picos de Bragg utilizados para o estudo das regras de extinção no BA, indica que a 150 K começam a observar-se pequenas deformações ou reorientações das suas unidades moleculares, que constituem uma manifestação de efeitos precursores das transições de fase a baixas temperaturas. Embora de um modo indirecto, alguns resultados experimentais apontam para a existência de agregados polares abaixo de 150 K. O comportamento em temperatura do modo relaxacional observado nos espectros de difusão Raman e de reflectividade no infravermelho indicam que a transição de fase a 119 K é predominantemente do tipo ordem-desordem, associada à ordenação dos protões nas pontes de hidrogénio mais fortes. Contudo, há evidências para que a transição de fase a 119 K tenha também um pequeno caracter de deslocamento, sem que no entanto tivesse sido detectado um modo mole. A transição de fase a 107 K parece estar associada a pequenas deformações da molécula de arseniato de betaína. A polarização espontânea é nula na direcção a, pelo que ..

Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group at ambient conditions. It is isostructural with the model soft-mode driven ferroelectric PbTiO3, but differs from it by the so-called 'supertetragonal' elongation of its unit cell. In this paper, we report a combined study of the lattice dynamics of PbVO3 by Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical (TO) phonon modes are identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374 and 508 cm-1, A1 modes at 188, 429 and 874 cm-1 and B1 mode at 319 cm-1. The calculations confirm the experimental symmetry assignment and allow to obtain the longitudinal (LO) phonons wavenumbers. Besides, we analyze the mode eigenvectors in detail, in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. In spite of their differences in chemistry and strain, the phonon eigenvectors are found to be remarkably comparable in both compounds. We discuss the position of the ferroelectric soft mode in PbVO3 as compared to PbTiO3. A sizeable splitting of the B1+E modes appears as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V-O bond frequency in PbVO3 is also addressed.Comment: 11 pages, 8 figures, 6 Tables, Supplemental Information 3 figures and 6 Tables, under revie

Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group under ambient conditions. PbVO3 is isostructural with the model soft mode-driven ferroelectric PbTiO3, but it differs due to the so-called “supertetragonal” elongation of its unit cell. In this study, we investigated the lattice dynamics of PbVO3 based on Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical phonon modes were identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374, and 508 cm−1; A1 modes at 188, 429, and 874 cm−1, and B1 mode at 319 cm−1. The calculations confirmed the experimental symmetry assignment and allowed us to obtain the longitudinal optical phonon wavenumbers. In addition, we analyzed the mode eigenvectors in detail in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. Despite differences in chemistry and strain, the phonon eigenvectors were found to be highly comparable in both compounds. We investigated the position of the ferroelectric soft mode in PbVO3 compared with PbTiO3. Sizeable splitting of the B1+E modes appeared as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V–O bond frequency in PbVO3 was also addressed

Electrical properties of polypropylene-based composites melt-processed with as-grown carbon nanofibers

Electrical conductivity, dielectric permittivity, electrical modulus, and electrical impedance of polypropylene (PP) composites melt-processed with different contents of as-grown carbon nanofibers (CNFs) are studied. As expected, the electrical conductivity of PP/CNF composites increased as the incorporation of CNFs is raised in the polymer, yielding a maximum of ∼ 6 ×10−6 S m−1 for PP/CNF 3 wt. % composites. That enhancement relates to a gradual improvement of the dielectric permittivity as the incorporation of CNFs rises into the PP up to a maximum of ∼ 13 for PP/CNF 3 wt. % composites at 1MHz, which is attributed to the rise of the interface polarization effect. Moreover, the Cole-Cole model is used through the electrical modulus to analyze the effect of CNF contents on the dielectric relaxation of PP/CNF composites from which is deduced that the incorporation of CNFs increases their heterogeneity and relaxation times. The analysis gathered here aims at contributing to the understanding of the electric features of polymer composites filled with a type of CNFs, which are not subjected to any thermal post-processing method after their synthesis by chemical vapor deposition (CVD).This research was funded by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) co-financed by FEDER through the PT2020 program

Strain relaxation dynamics of multiferroic orthorhombic manganites.

Resonant Ultrasound Spectroscopy has been used to characterise strain coupling and relaxation behaviour associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3 and TbMn0.98Fe0.02O3 through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic colinear-sinusoidal/incommensurate/antiferromagnetic transition at ~41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ~150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy, Ea, of ≥ ~0.04 eV from a loss peak at ~80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ~10-6 s. Comparison with previously published data for Sm0.6Y0.4MnO3 confirms that this pattern may be typical for multiferroic orthorhombic RMnO3 perovskites (R = Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3 and TbMn0.98Fe0.02O3, but not for GdMnO3, yielded Ea ≥ ~0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ~0.1-1 ‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic - orthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients.This work was funded by EPSRC Grant No. EP/ P024904/1 (UK). RUS facilities were established through grants from the Natural Environment Research Council (Grants No. NE/B505738/1 and No. NE/F017081/1) and the Engineering and Physical Sciences Research Council (Grant No. EP/I036079/1) to MAC. The work at the University of Warwick was supported by EPSRC,UK through Grant EP/T005963/1

Electrical Properties of Polypropylene-Based Composites Melt-Processed with As-Grown Carbon Nanofibers

Electrical conductivity, dielectric permittivity, electrical modulus, and electrical impedance of polypropylene (PP) composites melt-processed with different contents of as-grown carbon nanofibers (CNFs) are studied. As expected, the electrical conductivity of PP/CNF composites increased as the incorporation of CNFs is raised in the polymer, yielding a maximum of ∼ 6 ×10−6 S m−1 for PP/CNF 3 wt. % composites. That enhancement relates to a gradual improvement of the dielectric permittivity as the incorporation of CNFs rises into the PP up to a maximum of ∼ 13 for PP/CNF 3 wt. % composites at 1MHz, which is attributed to the rise of the interface polarization effect. Moreover, the Cole-Cole model is used through the electrical modulus to analyze the effect of CNF contents on the dielectric relaxation of PP/CNF composites from which is deduced that the incorporation of CNFs increases their heterogeneity and relaxation times. The analysis gathered here aims at contributing to the understanding of the electric features of polymer composites filled with a type of CNFs, which are not subjected to any thermal post-processing method after their synthesis by chemical vapor deposition (CVD)

Strain relaxation dynamics of multiferroic orthorhombic manganites

Resonant Ultrasound Spectroscopy has been used to characterise strain coupling and relaxation behaviour associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3 and TbMn0.98Fe0.02O3 through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic colinear-sinusoidal/incommensurate/antiferromagnetic transition at ~41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ~150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy, Ea, of ≥ ~0.04 eV from a loss peak at ~80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ~10-6 s. Comparison with previously published data for Sm0.6Y0.4MnO3 confirms that this pattern may be typical for multiferroic orthorhombic RMnO3 perovskites (R = Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3 and TbMn0.98Fe0.02O3, but not for GdMnO3, yielded Ea ≥ ~0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ~0.1-1 ‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic – orthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients

Dielectric spectroscopy of melt-mixed polypropylene and pyrolytically stripped carbon nanofiber composites

In this work, pyrolytically stripped carbon nanofiber (CNF) polypropylene (PP) composites were synthesized following a scalable melt-mixing method, and the effects of CNF weight concentrations on the electrical conductivity, dielectric permittivity, electrical modulus and electrical impedance of PP/CNF composites were studied. Quite unexpectedly, the electrical conductivity of PP/CNF composites improved only slightly as the incorporation of CNFs was raised, yielding a maximum of ~10−10 S m−1 for PP/CNF 5 wt. % composites. The increase corresponded to a gradual improvement of the dielectric constant up to a maximum of ~9 for PP/CNF 5 wt. % composites at 1 MHz, which was attributed to the raise of interface polarization effect. Moreover, the Cole–Cole model was used to analyze the effects of CNF concentrations on the dielectric relaxation of PP/CNF composites, from which was deduced that the incorporation of CNFs increases their dielectric strength and relaxation times. The analysis gathered here aims to provide a better insight into the enhanced dielectric properties observed in low-conducting polymer composites filled with CNFs.A. J. Paleo gratefully acknowledges support from FCT-Foundation for Science and Technology by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) co-financed by FEDER through the PT2020 program and “plurianual” 2020–2023 Project UIDB/00264/2020

Discrimination of Benign and Malignant Lesions in Canine Mammary Tissue Samples Using Raman Spectroscopy: A Pilot Study

Breast cancer is a health problem that affects individual life quality and the family system. It is the most frequent type of cancer in women, but men are also affected. As an integrative approach, comparative oncology offers an opportunity to learn more about natural cancers in different species. Methods based on Raman spectroscopy have shown significant potential in the study of the human breast through the fingerprinting of biological tissue, which provides valuable information that can be used to identify, characterize and discriminate structures in breast tissue, in both healthy and carcinogenic environments. One of the most important applications of Raman spectroscopy in medical diagnosis is the characterization of microcalcifications, which are highly important diagnostic indicators of breast tissue diseases. Raman spectroscopy has been used to analyze the chemical composition of microcalcifications. These occur in benign and malignant lesions in the human breast, and Raman helps to discriminate microcalcifications as type I and type II according to their composition. This paper demonstrates the recent progress in understanding how this vibrational technique can discriminate through the fingerprint regions of lesions in unstained histology sections from canine mammary glands

A História da Alimentação: balizas historiográficas

Os M. pretenderam traçar um quadro da História da Alimentação, não como um novo ramo epistemológico da disciplina, mas como um campo em desenvolvimento de práticas e atividades especializadas, incluindo pesquisa, formação, publicações, associações, encontros acadêmicos, etc. Um breve relato das condições em que tal campo se assentou faz-se preceder de um panorama dos estudos de alimentação e temas correia tos, em geral, segundo cinco abardagens Ia biológica, a econômica, a social, a cultural e a filosófica!, assim como da identificação das contribuições mais relevantes da Antropologia, Arqueologia, Sociologia e Geografia. A fim de comentar a multiforme e volumosa bibliografia histórica, foi ela organizada segundo critérios morfológicos. A seguir, alguns tópicos importantes mereceram tratamento à parte: a fome, o alimento e o domínio religioso, as descobertas européias e a difusão mundial de alimentos, gosto e gastronomia. O artigo se encerra com um rápido balanço crítico da historiografia brasileira sobre o tema